Various techniques have been developed to facilitate communication of data signals over an associated communications path. The particular communications protocol employed generally depends on the transmission medium, the available bandwidth resources, and other design considerations. Regardless of the type of communications system being employed, noise, distortion and multipath interference are often introduced into data signals transmitted over an associated communications path, including both wired and wireless systems.
Multipath interference is the unwanted addition of one or more signals to a desired signal. Multipath signals are generated by reflectors such as buildings, cars, signs, etc., which the signal bounces off of generating a delayed, attenuated and phase shifted signal. At the receiving antenna, the net received field is the sum of all incoming electromagnetic energy including both the desired signal and the multipath signals.
FIG. 1 illustrates destructive multipath interference. FIG. 1 shows graphs of a desired or line of sight signal 101, a received signal 102 and a multipath signal 103. The multipath signal 103 is shown as a “worst case” multipath signal having a differential path delay of one half of a carrier period. The differential path delay is the difference in delay between the desired and/or line of sight (LOS) signal and a multipath signal. Because of multipath interference, the received signal 102 is approximately flat because the desired signal 101 is canceled by the multipath signal 103.
Communication systems generally have fixed stations or mobile stations. For fixed station communication systems, multiple antennas can be spread out and spaced apart so that multipath signals causing destructive interference at one antenna will not cause the same destructive interference at another antenna. This technique is commonly referred to as using spatial diversity. However, spatial diversity is not usually feasible for mobile station communication systems. For example, a wireless telephone system would not work well if a user was required to set up multiple antennas spaced apart from each other.
A number of methods, aside from spatial diversity, are used to reduce or prevent multipath interference. Included among these methods are quadrature amplitude modulation (QAM) with decision equalization feedback (DEF), direct sequence spread spectrum, frequency division multiplexing (FDM) and orthogonal frequency division multiplexing (OFDM). However, these methods all have problems dealing with multipath interference.
Using QAM with DEF requires a formidable amount of hardware and generally does not work well in dense multipath environments. Once a strong multipath reflection has cancelled the incoming signal, there is nothing remaining in the incoming signal with which to retrieve the desired signal. For signals which are only partially cancelled, an equalizer may provide enough signal to noise ratio improvement for the signals to be processed correctly. However, for strong multipath interference, QAM with DEF does not work well.
The spread spectrum approach will work even in the presence of worst case multipath interference from a single reflector, but requires that the spectrum be spread over a wide band. Generally, a bandwidth of at least twice the carrier frequency will work. Thus, a carrier frequency of 2.5 GHz would require a spread spectrum from 2.5 GHz to 7.5 GHz. Such large bandwidth requirements make this approach not practicable.
The strongest of the above approaches is OFDM, a form of frequency shift keying in which the frequencies transmitted are chosen to be orthogonal to one another. Thus, guard bands are not required to keep one channel from interfering with neighboring channels and delayed signals from other frequency channels will have little or no effect on the desired channel. However, OFDM is problematic in that once a signal has been cancelled by a reflection or multipath interference, the signal is gone unless you apply coding and decrease throughput. OFDM requires coding and time scrambling to get the overall error rate low and thus decreases the net information flow by the additional overhead. In areas with significant multipath signals, a network controller at the base station is generally used to improve the performance of OFDM by dynamically allocating the frequencies to be used for each client transceiver by using special signals such as training signals. The network controller uses the response from the subscribers to the training signals to identify frequencies encountering attenuation and allocates working channels in place of nonworking channels.
However, multipath interference can still break OFDM down. Spatial diversity can be combined with OFDM, referred to as vector orthogonal frequency division multiplexing (VOFDM) to further reduce multipath interference. VOFDM requires that the antennas be positioned so that a deep multipath fade at one antenna will be received as a strong signal at another antenna. By using VOFDM, even though the received signal at one of the antennas may be severely attenuated, in many cases the received signal from the same channel out of the other spatially diverse antenna will be a strong useable signal. However, VOFDM systems are highly complex and require multiple spatially diverse antennas and multiple receiver RF sections, which is costly for fixed communication systems and not feasible for mobile communication systems.